Various beam spreads are required for stage lighting. Zoom lenses are complicated, expensive and not free of maintenance problems. Known fixed lens systems do not take advantage of the best optical combinations to project the maximum intensity of light. These known devices characteristically are designed so that metal patterns or masking shutters may be adjusted or inserted within the system so that their shadow edge is projected against a remote surface. Such known devices may include one simple plano-convex lens or two such lenses placed together with the plane sides on the outside. The two element system provides a shorter focal length than is possible with either lens individually, and, in fact, a shorter focal length than is possible with any single lens. The disadavantage with the two element system is that the light absorption is doubled, weight is high, and risk of fracture under thermal stress is great.
The primary object of this invention is to overcome these disadvantages and to provide a versatile lens casting or housing capable of providing various beam widths by different simple combinations of lenses.